KTiOAsO4 and related compounds are of interest because of their electro-optical properties. The thermal stability of the material is important both for its potential use as a device and in determining suitable conditions for its crystal growth. The stabilities and melting points of MTiOASO4, where M is K, Rb or Cs, and selected combinations thereof, have been investigated in oxygen and nitrogen using simultaneous thermogravimetry-differential thermal analysis (TG-DTA). The melting temperatures were observed to correlate with the ionic radius of M+. The materials are highly anisotropic and the thermal expansion of KTiOASO4 is reported over the temperature range from -170 to 950-degrees-C based on thermodilatometry (TD) of an oriented single crystal. The second order phase transformation at 861-degrees-C was much more evident in TD than in DTA.